1. Field of the Invention
The present invention relates to a continuous process for producing high molten viscoelastic polypropylene. More particularly, the present invention relates to the process for producing polypropylene having a wide molecular weight distribution and suited for post-finishing sheets and blow molding which comprises polymerizing propylene at multiple stages using two or more polymerizers connected in series.
The present invention further relates to a continuous process for producing high molten viscoelastic ethylene-propylene copolymers. More particularly, the present invention relates to the process which comprises performing a polymerization step (i) of propylene as a main component in a first to second tanks or a tank(s) subsequent thereto of polymerizers consisting of at least 3 connected in series and performing a polymerization step (ii) in which ethylene is used in a relatively large amount in a third tank or a tank(s) subsequent thereto.
The copolymers obtained by the processes of the present invention have more excellent impact strength than those obtained by known methods and are suited for post-finishing sheets, blow molding and injection molding.
2. Prior Art Statement
Sheets produced by finishing and molding known general-purpose polypropylene involve various defects below. These are such problems that (1) upon molding for secondary finishing, sagging of the sheets is rapid, allowance for finishing conditions is narrow, a molding efficiency is poor, etc.; and in addition, (2) in broad sheets, the aforesaid sagging is serious, a thickness of post-finished articles tends to be non-uniform and wrinkles due to sags laid on one another tend to occur, etc. For these reasons, molded articles of a small size can only be produced.
On the other hand, in the case of finishing known general-purpose polypropylene by blow molding, the following problems encountered. Namely, (1) due to serious sagging of parison upon molding, a thickness of molded articles become non-uniform. Thus, blow molding is applicable only to molded articles of a small size; (2) when polypropylene having a high molecular weight is used in order to prevent the sagging described above, not only dangers of causing poor fluidity of melt, an increased load upon molding, an increased loss of energy and other mechanical troubles increase but chalk mark of actually molded articles is remarkable and their commercial value is lost.
In order to improve the aforesaid sheet formability and blow formability in accordance with the use of polypropylene, low density polyethylene, etc. is incorporated in polypropylene, for example, in Published Examined Japanese Patent Application No. 80614/72 and Published Unexamined Japanese Patent Application No. 8848/75. However, articles molded from such a mixture tend to cause chalk marks; in order to prevent the chalk marks, intensive kneading is required upon melting and in this case, the practice of these inventions is restricted in view of selecting a kneader and power consumption. In addition, the products obtained in accordance with the inventions also involve a problem of decrease in rigidity. Further Published Unexamined Japanese Patent Application Nos. 91954/78, 185336/82, 187337/82, 7439/83, etc. propose methods for kneading polypropylenes having different molecular weights using a granulator. However, articles molded from mixtures in accordance with these inventions tend to cause chalk marks more seriously than those from the mixture of low density polyethylene described above and, the practice of these inventions is restricted from aspects of the kneading process and choice of difference in molecular weight.
In order to dissolve the foregoing problems involved in the formability of general-purpose polypropylene, many proposals have been made also on a method for broadening a molecular weight distribution of the product by multiple polymerization, for example, in Published Unexamined Japanese Patent Application Nos. 185304/82, 190006/82, 7405/83, 7409/83, 172507/84, etc. According to examples of these inventions, polypropylene having different molecular weights is formed at multiple steps by batch polymerization in most of them; however, the batch polymerization encounters a defect that productivity per a unit facility is low as a commercial process, because this method makes so called idle time such as charging raw materials, withdrawing products, etc.
Further in the inventions of Published Unexamined Japanese Patent Application No. 185304/82 and described thereafter, continuous polymerization is also mentioned to the effect that (a) when the order of stagewise production is in accordance with the combination of high molecular weight articles to low molecular weight articles, a necessary difference in molecular weight can be achieved merely by adding hydrogen at a later stage of multiple polymerization so that such is preferred as a process but conversely, (b) when the order is in accordance with the combination of low molecular weight articles to high molecular weight articles, unnecessary hydrogen must be removed from a tank in which a polymerization mixture is retained, by lowering the pressure or deaeration, etc. prior to production of high molecular weight articles and therefore, (b) is inferior in processability to (a) described above.
However, according to duplication and investigations of the present inventor, in the case of production through the order of high molecular weight articles to low molecular weight articles mentioned to be preferred in the prior art supra, a problem encounters that a melt flow rate (hereafter referred to as MFR) of the high molecular weight portion can be measured only with difficulty or it is impossible when MFR is low, resulting in an obstacle for controlling operation (note: it is possible to measure viscosity [.eta.] but much time is required for the measurement so that this is not a practical method as a means for control of operation).
Furthermore, the polypropylene produced in multiple steps in the order of high molecular weight to low molecular weight results in an abnormally large difference between MFR value of the powders prior to granulation and that of pellets obtained by granulation (note: MFR value of the powders is low) and, it has been noted that there are problems in controlling the difference in molecular weight at each step as the multiple polymerization and in controlling MFR values of the products.
On the other hand, crystalline polypropylene involves a problem that impact strength, in particular, impact strength at low temperatures is low, while it has excellent physical properties in rigidity, heat resistance, etc. From this aspect, area of practical use has been limited. In order to improve the defect, many proposals have been made on a process for block copolymerizing ethylene or other .alpha.-olefins. These many proposals have been made in, for example, Published Unexamined Japanese Patent Application Nos. 142652/75, 8094/77, 34112/82 etc. In these proposals, however, after polymerization 1 of propylene as the major component is performed, hydrogen is removed and polymerization 2 of monomers containing ethylene in a relatively large amount is performed, whereby some measures are elaborated to broaden the molecular weight distribution.
In general, in the case of adopting multiple continuous polymerization in block copolymerization, some distribution (note: it is considered to be close to a full mixing tank distribution) in residence time of each catalyst particle in each step so that the product becomes an aggregate of polymer particles in which proportions of the polypropylene content and the polyethylene content (note: a portion containing ethylene in a relatively large amount) vary depending upon time and, based on this heterogeneity, defects generate in quality.
Particularly in the continuous process for producing copolymers of the present invention later described in detail, a molecular weight difference is given in every stage in step (i) of the multiple polymerization of propylene as the major component so that when the prior art technique is adopted as it is, the molecular weight difference in each polymer particle is broadened than in conventional block copolymerization and problems due to heterogeneity could be more remarkable.
Many methods for improving the foregoing problems of the multiple continuous polymerization have also be proposed. For example, in Published Unexamined Japanese Patent Application Nos. 48916/83, 116716/80, 69215/83, etc., there is proposed a method in which a slurry released from a polypropylene polymerization part (polymerization step 1) is classified by cyclone and the finely divided particles are returned to the polypropylene polymerization part. However, classification based on polymer graininess is not necessarily identical with a residence time distribution and the effect is thus insufficient. In Published Unexamined Japanese Patent Application Nos. 195718/82, 29811/83, etc., there are also proposed some methods in which supply of a catalyst to a polymerizer and withdrawal of the slurry from the polymerizer are made intermittently and a part of the catalyst going into a polyethylene polymerization part (polymerization step 2) is reduced while the residence time is short. These methods involve a problem that the polymerization is unstable.
In addition, like embodiment 8 of the present invention later described, there are also proposed some methods in which the slurry released from the polypropylene polymerization part is treated with an electron donating compound, etc. thereby to selectively inactivate the catalyst particle discharged in a short residence time.
For example, in Published Unexamined Japanese Patent Application Nos. 145115/82 and 115417/80, various electron donating compounds are proposed but the compounds in the range used in the examples are insufficient to achieve the objects of the present invention later described.
Based on the results of various investigations to dissolve the problems described above, the present inventors proposed an invention of Japanese Patent Application No. 283728/85. However, it was troublesome in that the process was somewhat complicated because at least four polymerizers are required, there are limitations to a reaction weight ratio and polymerization conditions in each polymerizer, etc.
As a result of various investigations to solve the technical problems described above, the present inventors noted that when two or more polymerizers were connected in series, a catalyst and hydrogen were fed only in a first tank and propylene was polymerized, the polymerization mixture in the first tank was sequentially transferred in a polymerizer at a later stage and accompanied thereby, the reaction progress, catalyst concentration and hydrogen concentration at later stages were reduced in order so that polymers having a high molecular weight can be produced and polypropylene having a broad molecular weight distribution could be obtained as the final product. Based on this finding, an invention of Japanese Patent Application No. 264593/85 was proposed. In the process described above, however, problems encountered that operation and control were complicated because three or more polymerizers were required, there are limitations to a reaction weight ratio and polymerization conditions in each polymerizer, etc.